1. Field of the Invention
The present invention relates to a polishing composition useful for finishing polishing of the surface of a magnetic disk substrate in the preparation of a substrate for a magnetic disk to be used for a memory hard disk, i.e. a memory device useful for e.g. a computer. More particularly, the present invention relates to a polishing composition to be used for the preparation of a memory hard disk represented by e.g. a Ni--P disk, a Ni--Fe disk, an aluminum disk, a boron carbide disk or a carbon disk, particularly a polishing composition which provides a high stock removal rate in a polishing process for finishing a highly specular surface with a good surface roughness and which, at the same time, is applicable to a production technique for obtaining an excellent finished surface which is useful for a magnetic disk device having a large capacity and a high recording density. Further, the present invention relates to a method for polishing a memory hard disk employing such a polishing composition.
2. Discussion of Background
There have been continuing efforts for miniaturization and larger capacity for memory hard disks to be used for magnetic disk devices which are one of memory media for e.g. computers, and magnetic media are being changed from conventional coating type media to thin film media prepared by sputtering, plating or other methods.
A disk substrate (hereinafter referred to simply as "a substrate") which is most widely used at present, is one having an electroless Ni--P plating film formed on a blank material. Here, the blank material is one obtained by fairing an aluminum or other base plate by lathe processing by diamond turn, lapping by means of a PVA grindstone prepared by fixing SiC grinding material or other methods for the purpose of parallelization or planarization. However, by such various fairing methods, a relatively large waving can not completely be removed. And, the electroless Ni--P plating film will be formed along the waving on the blank material. Accordingly, such a waving will remain also on the substrate, and nodules or large pits will sometimes be formed. Here, the "nodules" are bulges having a diameter of at least about 50 .mu.m, which are formed by bulging of a plating surface at such portions that impurities have been taken into the Ni--P plating film. The "pits" are dents formed by polishing on the surface of the substrate, and "fine pits" are dents having a diameter of less than about 10 .mu.m, among them.
On the other hand, along with the increase in the capacity of memory hard disks, the surface recording density is increasing at a rate of a few tens % per year. Accordingly, the space on a memory hard disk occupied by a predetermined amount of recorded information, is narrower than ever, and the magnetic force required for recording tends to be weak. Accordingly, for recent magnetic disk devices, it is required to minimize the flying height of the head, which is a space between the magnetic head and the memory hard disk, and at present, the flying height of the head is reduced to a level of not higher than 0.15 .mu.m.
Further, so-called texturing may sometimes be carried out to impart concentric circular scorelines to the substrate after polishing for the purposes of preventing sticking of the magnetic head for reading or writing information, to the memory hard disk and preventing non-uniformity of the magnetic field on the memory hard disk due to scorelines in a certain direction different from the rotational direction of the memory hard disk, formed on the substrate surface by polishing. Recently, for the purpose of further reducing the flying height of the head, light texturing is carried out wherein the scorelines formed on the substrate are further reduced, or a non-texture substrate free from scorelines, is employed which is not subjected to texturing. The technology to support such a low flying height of the magnetic head has also been developed, and the reduction of the flying height of the head is being increasingly advanced.
When a memory hard disk surface has a waving, the head moves up and down following the waving of the memory hard disk which is rotated at a very high speed. However, if the waving exceeds a certain height, the head will no longer be able to follow the waving, and the head will collide against the substrate surface, thus resulting in so-called "head crush", whereby the magnetic head or the magnetic medium on the memory hard disk surface may be damaged, which may cause a trouble of the magnetic disk device, or which may cause an error in reading or writing information.
On the other hand, head crush may occur also when a micro protrusion of a few .mu.m is present on the memory hard disk surface. Further, when a pit is present on a memory hard disk, it is likely that information can not completely be written in, thus leading to a defect of information so-called a "bit defect" or failure in reading the information, which causes an error.
Accordingly, it is important to minimize the surface roughness of the substrate in the polishing step i.e. the step prior to forming a magnetic medium, and at the same time, it is necessary to completely remove a relatively large waving as well as micro protrusions, fine pits and other surface defects.
For the above purpose, it used to be common to carry out finishing by one polishing step by means of a polishing composition (hereinafter sometimes referred to as a "slurry" from its nature) comprising aluminum oxide or other various abrasives and water as well as various polishing accelerators. However, by the one polishing step, it has been difficult to satisfy all of the requirements for removing a relatively large waving as well as surface defects such as nodules and large pits on the substrate surface and for minimizing the surface roughness within a certain period of time. Accordingly, a polishing process comprising two or more steps, has been studied.
In a case where the polishing process comprises two steps, the main purpose of the polishing in the first step will be to remove a relatively large waving as well as surface defects such as nodules and large pits on the substrate surface, i.e. fairing. Accordingly, a polishing composition is required which has a high ability of correcting the above-mentioned waving and surface defects without forming deep scratches which can not be removed by polishing in the second step, rather than minimizing the surface roughness.
The purpose of polishing in the second step i.e. finishing polishing, is to minimize the surface roughness of the substrate. Accordingly, it is important that the polishing composition is capable of minimizing the surface roughness and capable of preventing formation of micro protrusions, fine pits or other surface defects rather than it has a high ability for correcting a large waving or surface defects as required for polishing in the first step. Further, from the viewpoint of the productivity, it is also important that the stock removal rate is high. So far as the present inventors are aware, in the conventional two step polishing, it was possible to obtain a substrate surface having a good surface roughness in the polishing in the second step, but the stock removal rate was very low and inadequate for practical production. The degree of the surface roughness is determined depending upon the process for producing the substrate, the final recording capacity as a memory hard disk and other conditions. However, depending upon the desired degree of surface roughness, a polishing process comprising more than two steps, may be employed.
For the above purpose, particularly in finishing polishing in two steps, it has been common to carry out polishing by means of a polishing composition prepared in such a manner that aluminum oxide or other abrasive is thoroughly pulverized and adjusted for proper particle size, water is added thereto, and aluminum nitrate or various organic acids and other polishing accelerators are incorporated thereto, or a polishing composition comprising colloidal silica and water. However, the polishing by means of the former polishing composition had a problem that the balance between the mechanical component and the chemical component was poor, and micro protrusions or fine pits tended to form. The polishing by means of the latter polishing composition had a problem such that the stock removal rate was so low that it took a long time for polishing, and the productivity was low, roll off (or "dove off") as an index of sagging of an end face of the substrate tended to deteriorate, or washing after the polishing tended to be difficult.